Hybrid engine technology has demonstrated the capability of reducing emissions by capturing energy during braking that would otherwise be lost. Hybrid technology has also demonstrated the capability of supplementing the engine power with additional torque response.
Application of exhaust aftertreatment systems, namely lean NOx trap (LNT) and particulate filters, requires specific control to regenerate these devices.
LNT aftertreatment systems require regular regeneration (e.g., up to one regeneration every 30 seconds). Regeneration requires that the gases delivered to the LNT be nearly free of oxygen. Historically, lean burn engines utilized exhaust-side supplemental fuel injection systems to reduce excess oxygen upstream of the LNT. From an efficiency standpoint, the supplemental fuel was wasted, as it did not contribute to engine output power. In-cylinder supplemental fuel injection is another method of supplying reductant to the LNT, but during the transition from lean to rich (and the return), excursions in engine power occur and are difficult to tune out. These excursions can perpetuate through the drivetrain and cause vehicle operating changes.
Particulate filters, catalyzed or not, increase the pressure drop across the units as they become loaded. The filters are normally regenerated when exhaust temperatures are high enough to oxidize the collected soot and hydrocarbons. There may be cases of extended light-load operation in which the exhaust temperatures are too low for regeneration to occur. Therefore, the pressure drop across the particulate filter increases greatly. Manufacturers are looking for active ways to regenerate the particulate filters.